A filling level sensor in the field of this invention is known from U.S. Pat. No. 5,895,848 and from German Patent DE 44 39 879 C1.
The functional principle of such a filling level sensor rests on the fact that the resonance frequency of the mechanical oscillator changes depending upon the density and, to a slight degree, upon the viscosity of a medium in which the oscillator is immersed.
The electrical resonant circuit of such a filling level sensor must be tunable within the limits of the anticipated resonance frequencies of the mechanical oscillator. When starting the filling level sensor, beginning with an initial frequency, the electrical resonant circuit is tuned to various frequencies in order to determine the current resonance frequency of the mechanical oscillator. A resonance frequency is then regarded as having been found if a 90° difference of phase is found between the phase of a motive force acting upon the mechanical oscillator and the movement of the oscillator. If no such frequency is found within the tuning range, it is assumed that the mechanical oscillator is blocked, e.g. by contact with a foreign object, and a malfunction of the filling level sensor is recognized.
The principle of error recognition will also fail if the tuning fork is detuned as a result of deposits on the tuning fork or mechanical damage to the tuning fork (as a result of erosion of the fork's tines, for example). In that case, the frequency that corresponds to this portion of the spectrum is detected as a resonance frequency of the mechanical oscillator. Its dependency upon the density and viscosity of the medium surrounding the mechanical oscillator can, however, be completely different in nature than that of an unhampered oscillator, making reliable monitoring of the filling level impossible.
In order to be able to operate a filling level sensor of the type alluded to at the outset, with various tuning fork oscillators that have different resonance frequencies, the oscillator must have a wide tuning range available. (Different resonance frequencies are caused by different tuning fork geometries or coatings, such as enamel, for example.) On the other hand, the probability that the oscillation of an oscillator whose movement is restricted exhibits a spectral component within the tuning range is all the greater the greater this tuning range is. This means the greater the distance between the frequencies of the individual oscillators for which such an oscillator may be used, the greater the risk that a partial blockage of the oscillator will remain undetected and that the filling level sensor could possibly provide false readings.